It is often desirable to determine the attitude of a spacecraft for payload pointing purposes. Attitude refers to angular orientation of the spacecraft with respect to three orthogonal axes. Satellites typically employ attitude determination apparatus for pointing a payload such as a telescope or antenna to a desired location on the Earth. Conventional attitude sensing apparatus may include a satellite receiver such as a Global Positioning System (GPS), ground tracking apparatus for determining the satellite ephemerides, and star or sun trackers for transforming reference measurements determined in spacecraft body coordinates into a stellar, or orbital frame of reference. Various methods have been used to process attitude sensor data to determine spacecraft attitude.
Several methods of determining spacecraft attitude using Extended Kalman Filter (EKF) based algorithms have been proposed. These estimate the spacecraft attitude and gyroscope rate biases using various attitude sensor data. For example, see E. J. Lefferts, et al., "Kalman Filtering for Spacecraft Attitude Determination," A.I.A.A. Journal on Guidance, Control and Dynamics, September-October 1982, pp. 417-429; A. Wu, "Attitude Determination for GEO Satellites," NASA Goddard 1997 Flight Mechanics Symposium, Greenbelt, MD, May 19-21, 1997.
The EKF is an established estimation method for attitude determination. In particular, the Kalman filter provides optimal noise attenuation performance for both process and measurement noise. EKF filtering is ideally suited for systems wherein disturbances are white noise processes. A steady state Kalman filter is a simple (fixed gain) estimator for state dynamics and measurement equations which are time-invanrant. If either the state dynamics or measurement equations vary with time, however, the Kalman filter gains become time-varying.
To reduce the computational complexity of such a system, a fixed gain filter approach can be used such as a TRIAD-based system. The TRIAD method of attitude determination is described in M. D. Shuster et al., "Three-Axis Attitude Determination From Vector Observations," Journal of Guidance and Control, vol. 4, no. 1, January-February 1981. A fixed gain approach to attitude determination reduces the computational complexity of the system, however, performance significantly deviates from the optimal solution provided by an EKF design because it ignores the time-varying measurement geometry in the filter design.
One problem with known attitude determination techniques is that the rate of successful attitude acquisition is lower than desired. That is, a proper orientation may not be obtained. If this occurs, then the process may have to be run again. This may result in costly delays. Another problem with attitude acquisition is that the time for acquisition is relatively great. This also increases costs due to unavailability of the satellite.